EBG devices are devices generally having an ability to suppress and filter electromagnetic energy. EBG devices are often used to help suppress switching noise and electromagnetic radiation in printed circuit boards (PCBs) and packages containing electronic devices. Such devices are also sometimes used to improve the performance of planar antennas by reducing cross-coupling between antenna array elements through surface waves, and by suppressing and directing radiation. EBG devices can be useful in other active and passive devices and applications such as oscillators, waveguides, transmission lines, amplifiers, filters, power combining circuits, phased arrays, mixers, and microwave components and devices.
A typical EBG device generally has a periodic structure, such as for example, a lattice, that is made up of periodic perturbations. These periodic perturbations, also known as vias, can take the form of holes or dielectric or metal rods or posts. Often an EBG device takes the form of a uniform substrate material with metal on both sides creating a parallel plate. The substrate between the parallel plates is typically loaded with metal or dielectric rods or patches that form the periodic perturbations.
FIG. 1A provides an example of a conventional EBG device 50 located in a printed circuit board (PCB) 62. FIG. 1B provides an enlarged view of the EBG device 50. As shown, EBG device 50 has a dielectric layer 52 positioned between two ground planes 54 and 54a. Embedded in dielectric layer 52 are conductive vias 56 in a regular periodic pattern. Conductive vias 56 are typically formed from metal or a metal alloy. EBG device 50 is also shown having a coplanar waveguide input 58, and a coplanar waveguide output 60. In operation, the periodic pattern of conductive vias 56 acts to filter the coplanar waveguide input 58 before the signal is output at the coplanar waveguide output 60.
A typical EBG device 50 functions to block or suppress the propagation of electromagnetic radiation that falls within a certain defined frequency band known as a stopband or bandgap. The EBG device 50 can be characterized by its stopband/bandgap characteristics. These can include the width of the stopband/bandgap and the location in the frequency spectrum of the stopband/bandgap. For a given EBG device 50, the characteristics of the stopband/bandgap are generally determined by the physical characteristics and location of the periodic perturbations or conductive vias 56 in the device. The overall effect of the conductive vias 56 in an EBG device 50 is to create a filter that blocks electromagnetic energy in a certain frequency range from propagating in the substrate and on the surface of the substrate. Characteristics of the perturbations, or conductive vias 56, that can determine the bandgap characteristics include the spacing of the perturbations, the size of the perturbations, and the material used to create the perturbations. By choosing certain materials, sizes, and locations, the width and frequency location of the bandgap can be selected. FIG. 1C generally illustrates the transmission characteristics associated with the conventional EBG device 50. As can be seen, the conventional EBG device 50 will typically pass certain frequency ranges (those above and below the bandgap), and will attenuate frequencies that fall within the bandgap. As seen in FIG. 1C, the bandgap is bounded on the high end by an upper bandgap frequency above which signals are not significantly attenuated.
Conventional EBG devices discussed above can also be formed to allow some frequencies of electromagnetic energy within the bandgap to propagate. This is commonly accomplished by including defects, called defect resonators, in the EBG structure when it is manufactured. These defect resonators are interruptions or defects in the symmetry of the otherwise regular pattern of periodic perturbations 56 in the EBG device 50. For example, in an EBG device 50 including a periodic pattern of perturbations that are conductive vias 56, a defect could be formed by not including one of the conductive vias in the periodic pattern when the EBG device is manufactured. In another example involving a single substrate plane with a periodic pattern of via apertures filled with a dielectric material, a defect could be formed by not filling one of the via apertures.
In operation, a defect resonator in an EBG device 50 typically creates an area of resonance in the EBG device 50 by localizing energy within the structure, allowing transmission of a narrow frequency within the stopband or bandgap of the EBG device 50. In effect, an EBG device 50 formed with a defect resonator typically acts as a high-Q filter, suppressing frequencies within the bandgap except for those resonated by defects. FIG. 1D provides a general illustration of the frequency characteristics of the conventional EBG device 50 having a defect resonator. As can be seen, an EBG device 50 having a defect resonator will typically allow some frequencies within the bandgap to pass through the EBG device without being significantly attenuated. The frequencies within the bandgap at which signals pass through the EBG device 50 having a defect resonator without being significantly attenuated are referred to as resonant frequencies.
Although characteristics of EBG devices with and without defect resonators can be selected prior to the manufacturing of the structures, manufacturing process imprecision, process tolerance limitations, and manufacturing cost tradeoffs can make it difficult to manufacture EBG devices having high upper band gap frequencies and high resonant frequencies to provide for desired performance in high-frequency applications. It is therefore desirable to provide for a bandgap devices, and methods for producing such devices, that can achieve higher upper bandgap frequencies and resonant frequencies without requiring the use of atypical, expensive manufacturing processes.